I looked through your logs, and the damage over time graph shows that you don't gain much damage at all during execute range. Upon further investigation, it seems that you do not save your cooldowns - Demon Soul, second pot, 4pc, and so on, for execute - these buffs all expired shortly before you cast your first Drain Soul channel. Properly lining up your cooldowns will make a huge difference in your numbers.

Additionally, it seems that you only used your 4-pc four times, despite the fight length easily accomodating six uses. Ensure that you are maximising your cooldown usage - 4pc buff incidentally lines up well with power torrent and every other Lightweave.

Do note that your logs are bugged due to Ultraxion's phase-shift mechanic, meaning that you could have been using cooldowns and I can't see it on the log because the logger was in a different phase than you when you used said CD. For the most accurate result, always log this fight yourself.